Test leak units are used for calibrating leakage flow meters. A test leak unit comprises a container in which a volume of a test gas or a test gas mixture is kept at a controlled pressure. The chamber includes an outlet connected to a capillary or any other defined test leak leading into the surroundings. For pressure control purposes, the chamber comprises a membrane which is supported by a spring, and which, upon deflection, actuates a control valve which switches on and off a gas-supplying pressure source.
Hydrogen may not be used as a pure gas in test leak units due to its explosiveness. Therefore, normally an oxygen-free hydrogen/nitrogen mixture is used as a test gas mixture (forming gas). However, the membrane sealing the chamber of the pressure control valve is not gas-impermeable. It rather offers different permeabilities to different gases. Consequently, hydrogen escapes more quickly from the chamber than nitrogen such that the nitrogen concentration in the chamber increases. Normally, the test gas mixture comprises 95% N2 and 5% H2. The hydrogen content can amount to up to 10%. This leads to an explosion risk.